This invention relates to both a method and apparatus (as illustrated in FIG. 1) for, particularly, the noninvasive determination of those parameters currently reported on a standard clinical arterial blood gas report. The parameters typically reported are hydrogen ion concentration (pH), partial pressure of carbon dioxide (PCO.sub.2), partial pressure of oxygen (PO.sub.2), bicarbonate concentration ([HCO.sub.3.sup.- ]) and oxygen saturation (O.sub.2 sat.).
Arterial blood gas determination is the cornerstone of diagnosis and management of cardiopulmonary disease in the critically ill patient. As effective oxygenation and maintenance of acid-base balance in such a patient is necessary for survival, measurement of arterial blood gases is typically the most frequently ordered laboratory test in a hospital's intensive care unit. In a patient with respiratory failure, the physician uses the results of blood gas analysis to optimize such a patient's oxygenation and acid-base status. Specifically, decisions regarding oxygen administration, titration of positive end expiratory pressure (PEEP) and minute ventilation are made, at least in part, on the results of arterial blood gas analysis. Repeated determinations are made over time to monitor the progression or remission of cardiopulmonary pathophysiology and to guide efforts at weaning patients from mechanical ventilatory support.
The standard arterial blood gas report contains the following information: pH, PCO.sub.2, PO.sub.2, [HCO.sub.3.sup.- ], and O.sub.2 saturation. The pH and PCO.sub.2 provide valuable information regarding acid-base and ventilation status. The bicarbonate level provides additional information on acid-base balance which allows the physician to determine whether an acid-base abnormality is respiratory or metabolic in origin. The two other indices, PO.sub.2 and O.sub.2 saturation, reflect the amount of oxygen present in the patient's blood.
At the present time standard clinical practice requires arterial puncture for procurement of an arterial blood sample. The arterial puncture is painful to the patient and associated with a variety of complications. Minor complications include arteriospasm, localized internal bleeding (i.e. hematoma), transient occlusion of the artery and temporary loss of sensation in the distribution of the median nerve. Major complications are infrequent, but include hemorrhage and severe vascular occlusion secondary to intraluminal clot formation. On rare occasions, gangrene has necessitated the amputation of a finger or a hand. When multiple samples are required over a relatively short period of time, an indwelling arterial catheter may be useful, the insertion of which can also be painful and which has complications such as described above. Although a standard indwelling arterial catheter allows for repeated sampling, it does not allow for continuous arterial blood gas monitoring.
The necessity, as well as associated complications, of blood gas monitoring are also present when monitoring infants, especially premature infants. The ventilation of premature infants is especially difficult due the immaturity of their lungs. Thus, to optimize the infants' chances for survival, the pediatrician or neonatologist must obtain multiple arterial blood gas determinations. Arterial puncture for procurement of the required arterial sample from an infant is typically more difficult than in adults due to the smaller size of the arteries.
In addition to the foregoing, the process for analyzing the arterial blood sample is lengthy, requires multiple personnel and is not continuous. Immediately after withdrawal, the arterial blood sample is placed on ice to inhibit red blood cell metabolism, which metabolism would alter the sample's blood gas parameters and lead to an incorrect measurement of the patient's blood gas values. The sample is then transported to the clinical chemistry laboratory in the hospital where it is logged in. Next, the sample is then analyzed by conventional electrochemical techniques. Finally, the results are entered in the hospital computer and made available to the physician for interpretation.
Due to the important clinical role of the information obtained by arterial blood gas analysis in the management of critically ill patients and the drawbacks of conventional clinical analysis, a number of alternative technologies have been proposed to measure one or more blood gas parameters. The prior art is quite diverse but can be divided into 7 major categories.
1. Invasive colorimetric technologies requiring direct contact between the colorimetric substances and arterial blood. These technologies measure pH, PO.sub.2, and PCO.sub.2.
2. Separation of the blood gases by semipermeable membranes, with subsequent concentration determination by absorption spectroscopy. The technology is limited to the measurement of PO.sub.2, and PCO.sub.2.
3. Electrode devices for measurement of PO.sub.2, and PCO.sub.2 in peripheral skin. This technology has been commercially developed by Radiometer, Denmark.
4. Invasive transistor devices for measurement of pH, and PCO.sub.2 in blood.
5. Semi-invasive techniques for measurement of pH, PCO.sub.2 and PO.sub.2 in peripheral skin.
6. Optical or spectroscopic determination of PCO.sub.2 and PCO (partial pressure of carbon monoxide) in tissue;
7. Pulse oximeters for noninvasive measurement of arterial blood oxygen saturation (O.sub.2 sat.).
The terminology used in clinical medicine, technical publications and the prior art patents is not always consistent. In the clinical practice of medicine the term "blood gas" is specific for the determination of certain factors in arterial blood. The determination of blood gases in venous blood is of little or no clinical significance and is not standard practice. Technically the term "blood gas" is misleading as not all of the components listed on a standard laboratory blood gas report are specific for gas measurements. PH and [HCO.sub.3.sup.- ] are concentration measurements of non-gaseous substances. In this application "blood gas parameters" are pH, PCO.sub.2, PO.sub.2, or [HCO.sub.3.sup.- ] and O.sub.2 saturation.
The foregoing parameters are not always measured in arterial blood; they can be measured in tissue and skin. Thus, it is important to distinguish between measurement in arterial blood, tissue and peripheral skin, as their sensitivity and clinical utility differ. Blood gas measurements in arterial blood are currently the reference standard used in clinical medicine and the associated body of knowledge on how to interpret the results is extensive. Additionally there is a minimal time delay between a change in cardiopulmonary status and a corresponding change in arterial blood concentrations.
Several prior art methodologies measure certain blood gas parameters in the peripheral skin or tissue. Peripheral skin and tissue are differentiated in that skin is the outermost layer of the body, while tissue refers to the sum total of body mass existing between two skin surfaces. Thus, a tissue measurement will contain information on the dermal and epidermal layers of the skin, muscle, bone, fat, blood vessels and capillaries. Tissue measurements, if accurate, may be the measurements of highest clinical utility as such measurements define the acid-base and oxygen status of the peripheral tissue. The maintenance of normal physiology in these tissues is the goal when treating cardiopulmonary disease. However, as current clinical instrumentation does not measure tissue blood gas parameters, the practicing physician will be unfamiliar in how to interpret these measurements. Additionally there may be a longer time delay between changes in cardiopulmonary status and a corresponding change in the tissue.
Measurement of blood gas parameters in peripheral skin has limited clinical utility. When peripheral skin measurements are compared to arterial blood measurements, it has been found that the skin measurements of PO.sub.2 are not adequate unless local hyperemia of the skin to facilitate oxygen diffusion is achieved. Peripheral skin measurements are also less sensitive to acute changes in blood gas concentrations. Peripheral skin measurements have so far proven reliable only in babies. In adults, the major difficulty relates to the thickness and permeability of the skin.
To summarize, there are five blood gas parameters which can be determined in three different media, (1) arterial blood, (2) tissue, and (3) peripheral skin. A ranking of the current clinical utility of these measurements would be arterial blood, tissue, and peripheral skin a distant third.